The present invention relates to wireless communication systems. More specifically, the present invention relates to the reception of communication signals in wireless communication systems.
Signal synchronization is important in wireless telecommunication. In modem systems, there are various levels of synchronization, such as, carrier, frequency, code, symbol, frame and network synchronization. At each level, synchronization can be divided into two phases: acquisition (initial synchronization) and tracking (fine synchronization).
A typical wireless communication system, such as specified in the 3rd Generation Partnership Project (3GPP), sends downlink communications from a base station to one or a plurality of User Equipments (UEs) and uplink communications from UEs to the base station. A receiver within each UE operates by correlating, or despreading, a received downlink signal with a known code sequence. The code sequence is synchronized to the received sequence in order to get the maximal output from the correlator.
A receiver may receive time offset copies of a transmitted communication signal known as multi-path. In multi-path fading channels, the signal energy is dispersed over a certain amount of time due to distinct echo paths and scattering. To improve performance, the receiver can estimate the channel by combining the multi-path copies of the signal. If the receiver has information about the channel profile, one way of gathering signal energy is then to assign several correlator branches to different echo paths and combine their outputs constructively. This is conventionally done using a structure known as a RAKE receiver.
Conventionally, a RAKE receiver has several xe2x80x9cfingersxe2x80x9d, one for each echo path. In each finger, a path delay with respect to some reference delay, such as the direct or the earliest received path, must be estimated and tracked throughout the transmission. The estimation of the paths initial position in time may be obtained by using a multi-path search algorithm. The multi-path search algorithm does an extensive search through correlators to locate paths with a desired chip accuracy. RAKE receivers are able to exploit multi-path propagation to benefit from path diversity of transmitted signal. Using more than one path, or ray, increases the signal power available to the receiver. Additionally, it provides protection against fading since several paths are unlikely to be subject to a deep fade simultaneously. With suitable combining, this can improve the received signal-to-noise ratio, reduce fading and ease power control problems.
In the context of mobile UEs, due to their mobile movement and changes in the scattering environment, the delays and attenuation factors used in the search algorithm change as well. Therefore, it is desirable to measure the tapped delay line profile and to reallocate RAKE fingers whenever the delays have changed by a significant amount.
An important design problem of a RAKE receiver is how to accurately search and find multiple signal paths. There are several key parameters to be optimized for the receiver system, such as mean acquisition time, optimum threshold setting, probabilities of detection and false alarm, etc. One problem with a RAKE receiver is that the paths can disappear or may not be detected by a RAKE location process. Therefore, there exists a need for an improved receiver.
Another severe design problem of a RAKE receiver is that it is not always possible to separate the received energy into components due to distinct multipath components. This may happen, for example, if the relative delays of the various arriving paths are very small compared to the duration of a chip. Such situations often arise in indoor and urban communication channels. The problem is often referred to as the xe2x80x9cFat Finger Effect.xe2x80x9d
While there exists techniques for demodulating the data from Fat fingers, in order to apply such techniques the received energy belonging to a Fat finger must be identified. Unfortunately, typical RAKE correlators are designed to search for distinct single-path components in a multipath channel and are unable to perform this identification. Thus, there exists a need for a receiver capable of identifying the Fat fingers.
The present invention is directed to an improved telecommunication receiver for receiving wireless multi-path communication signals. A novel RAKE receiver and a time diverse integration system for the calculation of the relative power of received signal samples are provided. Preferably, the receiver is embodied in a UE or base station of a CDMA wireless telecommunication system, such as a 3GPP system.
In one aspect of the invention, the station has a receiver for processing communication signals, which includes a RAKE receiver having up to a predetermined number of RAKE fingers, for assigning and combining a plurality of different signal paths of received communication signals. In one example, the maximum number of RAKE fingers is five (5) of which up to one is a Fat finger. A Fat finger of the RAKE receiver implements a Fat finger demodulation algorithm that, for example, may be a conventional Adaptive Filter.
The receiver has a RAKE locator that determines signal paths based on windows defined by groups of consecutive signal samples. Windows are defined where samples within a window exceed a first power threshold. The RAKE locator designates a number of such windows, up to the number RAKE fingers, as candidate windows based on relative power of the samples within the determined windows.
Preferably, the RAKE locator defines windows based on a window power level determined by summing power levels of its group of samples. A window is defined when its power level exceeds the first power threshold. Preferably, the RAKE locator designates windows as candidate windows based on windows having the highest power levels. However, a window is not designated if it is to close to another candidate window, i.e. if more than a specified number of samples are included in another window having a higher power level. For example, each window can contain a group of 21 samples and the candidate windows can have no more than 16 common samples so that the candidate windows are separated from each other by at least 5 consecutive samples.
Window search circuitry analyzes candidate windows to determine if the power of samples of the candidate windows exceeds a second threshold. The window search circuitry designates a Fat finger candidate window when at least one of the candidate windows has a selected number of candidate samples that exceed the second threshold. Preferably, the window search circuitry designates only one Fat finger candidate window, that being the candidate window having the greatest power level that also has a selected number, preferably four (4), of candidate samples having power levels exceeding the second threshold. Candidate samples are those samples remaining after pruning consecutive samples that exceed the second threshold.
A RAKE finger allocator assigns candidate windows for processing by either a conventional type of RAKE finger or a Fat RAKE finger such that candidate windows that are not designated as a Fat finger candidate window are each assigned to a different conventional RAKE finger. Preferably, the RAKE finger allocator assigns any candidate window designated as a Fat finger candidate window to a Fat RAKE finger.
Methods for processing communication signals using a RAKE receiver having up to a predetermined number, for example five (5), of RAKE fingers, which combines a plurality of different signal paths of received communication signals, are provided. Signal paths are determined based on windows defined by groups of consecutive signal samples in which samples within a window exceed a first power threshold. Up to the predetermined number RAKE fingers of such windows are designated as candidate windows based on relative power of the samples within the determined windows. Candidate windows are analyzed to determine if the power of samples of the candidate windows exceeds a second threshold. A Fat finger candidate window is designated when at least one of the candidate windows has a second predetermined number of candidate samples exceeding the second threshold. Candidate windows are assigned for processing by either a first type of RAKE finger or a different second type of Fat RAKE finger such that candidate windows that are not designated as a Fat finger candidate window are each assigned to a different RAKE finger of the first type.
Preferably, windows are defined which have a power level, determined by summing power levels of its group of samples, which exceeds the first power threshold and candidate windows are designated based on windows having the highest power levels. However, a window is not designated as a candidate window if more than a specified number of samples are included in another window having a higher power level. For example, each group of samples can contain 21 samples and the specified number can be set as 16 such that only windows separated from each other by at least 5 consecutive samples are designated as candidate windows.
Preferably, only up to one Fat finger candidate window is designated, being the candidate window having the greatest power level which also has the selected number of candidate samples having power levels exceeding the second threshold. Candidate samples are samples remaining after pruning consecutive samples that exceed the second threshold.
Preferably, any candidate window designated as a Fat finger candidate window is assigned to a Fat RAKE finger that comprises an Adaptive Filter.
In a second aspect of the invention, the receiver is configured to process communication signals based in part on relative power of signal samples where relative power is calculated as a function of values corresponding to time diverse signal samples. A buffer is provided which stores at least values r(r) that correspond signal samples Sr, which define a set R of samples. R is a subset of X consecutively received signal samples S0 through SXxe2x88x921 that corresponding to values r(0) through r(Xxe2x88x921) The number of elements of subset R is less than X such that R contains at least two mutually exclusive subsets of consecutive samples {S0 through Si} and {Sj through SXxe2x88x921} Accordingly, R does not include sample Si+1 or Sjxe2x88x921. For convenience the buffer may store all values r(0) through r(Xxe2x88x921), but a substantially smaller buffer can be used if only the time diverse subsets of values represented by sample set R are stored.
A processor is operatively associated with the buffer for calculating relative sample power based on values r(r) that correspond to signal sample elements Sr of the selected subset R of X consecutively received signal samples. Values of samples not contained in R, such as values r(i+1) or r(jxe2x88x921) that correspond to signal sample elements Si+1 and Sjxe2x88x921, respectively, are not used in the calculation. Accordingly, relative power is calculated based on sample series representing at least two diverse time intervals.
Preferably, the processor is configured to calculate relative power utilizing a function based on an index set I comprised of mutually exclusive subsets of positive integers, such that, for each subset of I, a corresponding subset of R is utilized in calculating relative power.
Each pair of consecutive samples represents a sampling time interval t that corresponds to the sampling rate used in obtaining samples of a received signal. Preferably, at least two mutually exclusive subsets of the X consecutive samples exist that contain at least consecutive samples {Si+1 through Si+51} and {Sjxe2x88x9251 through Sjxe2x88x921}, respectively, and do not contain any elements of subset R. In such case, subset R is defined by at least three mutually exclusive subsets of consecutive samples, which represent groups of consecutive samples mutually offset in time by at least 50 times t.
Preferably, the processor is configured to calculate correlation power PkPN between a PN scrambling sequence and a received signal for a sample Sk based on:       P    k    PN    =            ∑              m        ∈        I                    xe2x80x83              ⁢          xe2x80x83        ⁢          "LeftBracketingBar"                        ∑                      n            =            0                                N            -            1                          ⁢                  xe2x80x83                ⁢                              r            ⁡                          (                              Nm                +                n                            )                                ⁢                                    c              *                        ⁡                          (                              Nm                +                n                -                k                            )                                          "RightBracketingBar"      
where N is a predefined constant and c(xc2x7) represents values corresponding to PN scrambling sequences. In order to limit processing time, index set I is preferably defined by no more than 150 elements. In one example, the index set I equals {0-9, 50-69, 100-199}, N is 256. This results in R being defined by three corresponding mutually exclusive subsets of consecutive samples that represent groups of samples mutually offset in time by more than 5000 times t.
A RAKE finger allocation block preferably includes the buffer and associated processor configured for time diverse integration so that correlation powers of samples Sk are calculated in the allocation block on a time diverse integration basis. However, implementation of time diverse integration can be similarly applied to other components where relative signal sample power is calculated.
Other objects and advantages of the invention will be apparent to those of ordinary skill in the art from the following detailed description.